The present invention relates to fluid flow meters. More particularly, the present invention relates to a fluid flow meter which includes a rotor that is magnetically rather than mechanically linked to a rotational measuring device in order to eliminate potential leak paths between the flow path and the surrounding environment.
In many industries, such as the oil, gas and petrochemical industries, fluid meters are used to accurately measure large amounts of fluid passing through a pipeline in order to facilitate selling or otherwise transferring custody of the fluid. One type of fluid meter commonly used for this purpose is a reciprocating blade positive displacement (PD) meter. Referring to FIG. 1, such meters commonly include a meter housing, a fluid inlet and a fluid outlet. A slotted cylindrical rotor is rotatably mounted in the housing. A cam is fixedly mounted to the housing within the rotor. A number of blades are slidably mounted in the rotor slots and are fitted with cam following bearings. As fluid is pumped through the meter, it impinges on the blades, forcing the rotor to rotate within the housing. As the rotor rotates, the cam causes the blades to reciprocate in and out of the rotor. In some meters the blades on opposite sides of the cam may be fixed to each other, such that they reciprocate as a unit.
In the meter in FIG. 1, a maximum of two of the blades are fully extended at any given time. The space between the housing, the rotor and the two extended blades is the measuring chamber. The space between the housing, the rotor and the inlet and outlet is filled with a block of material which prevents direct fluid flow from the inlet to the outlet, thus forcing all the fluid through the measuring chamber. The cam causes the blades to retract into the rotor as they approach and pass the block.
Because the volume of the measuring chamber is accurately known, a known volume of liquid passes through the meter for every ¼ revolution of the rotor. Thus, the total quantity of liquid passing through the meter can be calculated by accurately measuring the number of revolutions of the rotor.
In some prior art fluid meters the housing may be double walled, with the space between the walls allowed to fill with the process fluid. This arrangement insures that the inner wall is pressure balanced and is not distorted by the pressure. Such distortion can change the volume of the measurement chamber and thus degrade the accuracy of the meter.
Referring to FIGS. 2 and 3, an exemplary prior art reciprocating blade PD meter is shown to include a housing 10 and a cover 12 which is bolted to the housing. A mechanical counter 20 is mounted above the cover. A rotor 14 is rotatably mounted on a central shaft 16 inside the housing. In order to calculate the amount of fluid flowing through the meter, information on the rotation of the rotor 14 must be transmitted to the counter 20. A first set of gears 22 transmits the rotation of the rotor 14 to a secondary shaft 24. The secondary shaft passes through the cover 12, and packing gland 26 is provided between the cover and the secondary shaft. A second set of gears 28 transmits the rotation of the secondary shaft 24 to the mechanical counter 20. The mechanical counter may include an optical encoder.
One disadvantage of such mechanical transmissions is that the penetration of the secondary shaft through the cover creates a potential leak path through which the process fluid or volatile organic compounds (VOC's) may escape to the surrounding environment. Another disadvantage of mechanical transmissions is that they can limit the performance turndown ratio (i.e., the range of flow rates over which the meter gives repeatable results) of the meter.
Many reciprocating blade PD meters can be expensive to manufacture. Because the volume of the measuring chamber is so critical to the accuracy of the meter, the components which define the chamber must be manufactured to very tight tolerances. Also, it is critical to the operation and accuracy of the meter that the proper clearance be maintained between the housing and the tips of the blades when the blades are fully extended. Thus the blade assemblies must be uniform and centered in the rotor to very tight tolerances. For these reasons, many prior art meters are difficult to adjust or service in the field.
Another disadvantage of many prior art reciprocating blade PD meters is that they must be mounted in a particular orientation to avoid “cocking” of the blades within the rotor. Such changes in the blade position or orientation can change the meter factor, requiring recalibration of the meter. This lack of flexibility in mounting orientation can make it more difficult and expensive to design and install the meter. This can also be a problem for truck mounted meters when the truck is not parked on level ground.